A polylactic acid is known as a polymer which is transparent and has biodegradability and practical moldability. However, it has hard and brittle characteristics, and for this reason, industrial use thereof is limited. In order to ameliorate hard and brittle characteristics of polylactic acids, for example, Japanese Unexamined Patent Application, First Publication No. H08-199052 and Japanese Unexamined Patent Application, First Publication No. H08-283557 disclose a method in which a polyether or an aliphatic polyester formed from an aliphatic dicarboxylic acid and an aliphatic diol is added as a plasticizer.
However, even in each of the methods described above, with a common addition amount, impact strength of polylactic acids can be slightly improved. In order to provide sufficient flexibility, it is necessary to add a larger amount of a plasticizer. If such a larger amount of the plasticizer is added, there are problems in that thermal resistance may be degraded or bleed-out may occur.
In addition, Japanese Unexamined Patent Application, First Publication No. H09-137047 discloses a composition in which a plasticizer comprising a polyester-based block copolymer formed from a polymer with a high melting point composed of a polylactic acid and a polymer with a low melting point is added to a polylactic acid. However, the composition described above is also insufficient in view of impact resistance.
On the other hand, Japanese Translation of PCT International Application No. H08-501584 (U.S. Pat. No. 5,502,158) discloses that a plasticizer such as a polyester having a solubility parameter ranging from 7.5 to 16.5 (cal/cm3), and in particular, ranging from 9.0 to 11.0 (cal/cm3), exhibits good compatibility with respect to a polylactic acid, and has superior transparency. However, many plasticizers disclosed in the aforementioned publication are low molecular compounds. Therefore, there are problems in that during a step of adding a plasticizer to a polylactic acid or during molding with the obtained composition, the plasticizer may volatilize or the plasticizer may bleed out from the moldings.
Japanese Unexamined Patent Application, First Publication No. H10-316846 discloses that a plasticizer comprising an ester compound of a polylactic acid having a solubility parameter ranging from 10.6 to 11.6 exhibits good compatibility with respect to a polylactic acid, and has great plasticizing (softening) effects. However, in accordance with the aforementioned publication, many plasticizers supplied for research are compounds with a low molecular weight, and therefore, there are problems in volatilization of the plasticizer and in bleed-out from the moldings, as described above.